It has been known for some time that optical fibers can be advantageously used to amplify, by means of stimulated Raman scattering (SRS), radiation guided through the fiber. See, for instance, Optical Fiber Telecommunications, S. E. Miller and A. G. Chynoweth, editors, Academic Press, 1979, pages 127-133, incorporated herein by reference. Various optical fiber communications systems that use Raman amplification of the signal have been proposed. An exemplary system is disclosed, for instance, in U.S. Pat. No. 4,558,921.
Essentially all optical fiber used today for transmission purposes is silica-based fiber. Since silica, or silica lightly doped with other elements such as germanium, is known to have a non-zero Raman cross section, it is possible to use the normal transmission fiber also for amplification purposes, and systems of this type have been proposed. See, for instance, L. F. Mollenauer et al, IEEE Journal of Quantum Electronics, Vol. QE-22(1), pages 157-173 (1986), and U.S. Pat. No. 4,401,364.
Another approach to the design of an optical fiber communications system with Raman amplification is to use ordinary low-loss transmission fiber in conjunction with one or more lengths of Raman-active fiber that differ in composition from the low-loss transmission fiber. In this case, the signal amplification is not distributed over essentially the whole length of the transmission path but is substantially localized in the Raman-active fiber sections. The Raman cross section of the Raman-active fiber typically is substantially larger than the Raman cross section of the low-loss transmission fiber. A convenient figure of merit for Raman-active fiber is the relative Raman cross section .sigma..sub.r =.sigma..sub.RA /.sigma..sub.SiO.sbsb.2, where .sigma..sub.RA is the Raman cross section of the Raman-active fiber, and .sigma..sub.SiO.sbsb.2 is the maximum Raman cross section of pure silica glass.
The prior art knows some Raman-active optical fibers whose .sigma..sub.r is substantially greater than 1. For instance, E. Desurvire et al, Electronics Letters, Vol. 19(19), pages 751-753 (1983), report optical amplification of 1.24 .mu.m radiation in GeO.sub.2 --SiO.sub.2 single mode fiber. Since it is known that .sigma..sub.r of pure GeO.sub.2 is about 10, it is evident that the .sigma..sub.r of GeO.sub.2 --SiO.sub.2 fiber will be less than 10. Use of GeO.sub.2 --SiO.sub.2 fibers or GeO.sub.2 fibers is also taught in European patent application No. 0146262.
C. Lin, Journal of Optical Communications, Vol. 4(1), pages 2-9 (1983), discusses the design of optical fibers for, inter alia, optical amplification by SRS, and discloses that P.sub.2 O.sub.5 has a .sigma..sub.r of about 5.
Japanese Pat. No. 56-70683 discloses Raman-active optical fiber that comprises a SiO.sub.2 --P.sub.2 O.sub.5 --GeO.sub.2 glass core surrounded by a clad layer having the same constituents but lower refractive index, and Y. Durteste et al, Electronics Letters, Vol. 21(17), pages 723-724 (1985) report on Raman amplification in fluoride glass fibers.
Prior art Raman-active optical fiber typically has .sigma..sub.r that is substantially less than 15. Thus, any optical fiber system that uses prior art Raman-active fiber to amplify the signal has to comprise relatively long lengths of the Raman-active fiber and/or use relatively high pump power levels. Having available Raman-active optical fiber having larger Raman cross section than prior art fiber would permit the use of shorter Raman amplifier sections and/or lower pump power levels, resulting in lower system cost and complexity. This application discloses high Raman cross section fiber, together with apparatus that comprises such fiber.
Various nonsilica-based glass systems have recently been investigated to determine their suitability in ultra low loss transmission fibers. Among these were glasses containing GeO.sub.2 and heavy metal oxides such as Bi.sub.2 O.sub.3, Tl.sub.2 O, PbO, and Sb.sub.2 O.sub.3. See, for instance, K. Nassau et al, Journal of the American Ceramic Society, Vol. 65(10), pp. 486-491 (1982), and D. L. Wood et al, Applied Optics, Vol. 21(23), pp. 4276-4279 (1982). These investigations have led to the general conclusion that such heavy metal oxide glasses may have potential for low loss transmission waveguide fiber for the long wavelength region (e.g., 2-4 .mu.m) if ultra high purity glass can be prepared.